Ion Mobility Spectrometry (IMS) is a technique used to separate ions in terms of their ion mobility with respect to a drift/buffer gas. According to different separation manners, ion mobility spectrometers can be generalized into two types: separation by time and separation by distance. A traditional time-of-flight ion mobility spectrometer belongs to the type of separation by time, and a differential mobility spectrometry analyzer belongs to the type of separation by distance. The ion mobility spectrometers are limited by ion diffusion and therefore have poor resolution and sensitivity compared with a mass spectrometer. But ion mobility spectrometers can provide additional information based on a molecular collision cross section or operate as ion pre-separation apparatuses for mass spectrometer. The resolution of a differential mobility spectrometer can be improved by optimizing a geometric structure of the apparatus (for example, inclined fields, cross flow fields, and periodic focusing differential mobility spectrometers). The resolution of a time-of-flight mobility spectrometer can be improved by increasing the length of a drift tube, raising an applied voltage, and adding radial confining electric fields (for example, radio frequency ion funnels, radio frequency quadrupole fields, and direct current periodic electrostatic fields).
To further increase the resolution, the gas flow direction and electric field can oppose each other as described by Zeleny (Zeleny, J. Philos, Mag. 46, 120(1898)) in the parallel flow analyzer. The Zeleny apparatus consists of two parallel grids between which ions of a particular mobility could be balanced under the two opposing forces (gas flow and electric field). Several attempts have been made to achieve a parallel flow analyzer, including an inclined grid method, ion trap with gas flow present, orthogonal extraction ion mobility spectrometer, Loboda segmented quadrupole field with opposing gas flow, and parallel flow ion mobility spectrometer/later trapped ion mobility spectrometer proposed by Park. Experimentally, the only successful apparatuses have resulted from approaches of Loboda and Park, both using radio frequency quadrupole fields to confine ions radially. In U.S. Pat. No. 6,630,662 B1, Loboda used a uniform electric field, and the electric field increases slowly to sweep ions out against the gas flow. In U.S. Pat. No. 7,838,826 B1, Park used a non-uniform electric field to first separate ions having different ion mobilities, and then the electric field decreases slowly so that ions are swept out of the analyzer by the gas flow. Parks used a higher gas pressure than Loboda to achieve higher resolution, but his apparatus suffers from problems of a long analysis time and the limited number of ions per analysis.
To separate a continuous ion beam, an ion mobility filter can be used. A differential mobility spectrometer can separate ions with different mobilities along a direction perpendicular to a gas flow. In addition, a continuous ion beam can also be separated by setting a High pass/Low pass mobility filter. In U.S. Pat. No. 7,718,960 B2, Hashimoto et al. originally proposed such an apparatus including two ion drift regions. Each region has an electric field and a gas flow opposing from each other, and the directions of the gas flows in the two drift regions are opposite to each other. Gas in the apparatus is introduced perpendicularly from a connection region of the two regions. A schematic diagram of this apparatus is shown in FIG. 1A. In U.S. Pat. No. 9,281,170 B2, Parks also used the method of introducing gas perpendicularly, but the apparatus uses a radio frequency quadrupole field to confine ions radially. The schematic structural diagram of this apparatus is shown in FIG. 1B. Both Hashimoto and Parks High/Low pass filter apparatuses have a problem that the gas flow direction in the gas flow introducing region is not fixed and there exists obvious turbulence that will greatly impede the resolution and ion transmission of the apparatus.
Therefore, there is a need to further increase both the resolution and sensitivity of the ion mobility spectrometry. There is also a need for a mobility spectrometry apparatus that can achieve high resolution with a large ion capacity and can also operate as an ion mobility filter for a continuous ion beam while retaining high resolution and sensitivity.